Volatiles in Protoplanetary Disks

Volatiles in protoplanetary disks Klaus M. Pontoppidan Space Telescope Science Institute Colette Salyk National Optical Astronomy Observatory Edwin A. Bergin University of Michigan Sean Brittain Clemson University Bernard Marty Universite´ de Lorraine Olivier Mousis Universite´ de Franche-Comte´ Karin I. Oberg¨ Harvard University Volatiles are compounds with low sublimation temperatures, and they make up most of the condensible mass in typical planet-forming environments. They consist of relatively small, often hydrogenated, molecules based on the abundant elements carbon, nitrogen and oxygen. Volatiles are central to the process of planet formation, forming the backbone of a rich chemistry that sets the initial conditions for the formation of planetary atmospheres, and act as a solid mass reservoir catalyzing the formation of planets and planetesimals. Since Protostars and Planets V, our understanding of the evolution of volatiles in protoplanetary environments has grown tremendously. This growth has been driven by rapid advances in observations and models of protoplanetary disks, and by a deepening understanding of the cosmochemistry of the solar system. Indeed, it is only in the past few years that representative samples of molecules have been discovered in great abundance throughout protoplanetary disks (CO, H2O, HCN, C2H2, + CO2, HCO ) – enough to begin building a complete budget for the most abundant elements after hydrogen and helium. The spatial distributions of key volatiles are being mapped, snow lines are directly seen and quantified, and distinct chemical regions within protoplanetary disks are being identified, characterized and modeled. Theoretical processes invoked to explain the solar system record are now being observationally constrained in protoplanetary disks, including transport of icy bodies and concentration of bulk condensibles. The balance between chemical reset – processing of inner disk material strong enough to destroy its memory of past chemistry, and inheritance – the chemically gentle accretion of pristine material from the interstellar medium in the outer disk, ultimately determines the final composition of pre-planetary matter. This chapter focuses on making the first steps toward understanding whether the planet formation processes that led to our solar system are universal. 1. Introduction earliest pieces of firm evidence for abundant ices in planetary systems came from cometary spectroscopy, showing The study of the role of ices and volatile compounds in the photodissociation products of what could only be com- the formation and evolution of planetary systems has a long mon ices, such as water and ammonia (Whipple 1950). Fol- and venerable history. Up until the late 20th century, due to lowing early suggestions by e.g., Kuiper (1953), the pres- the lack of knowledge of exo-planetary systems, the solar ence of water ice in the rings of Saturn (McCord et al. 1971) system was the only case study. Consequently, some of the 1 and in the Jovian satellite system (Pilcher et al. 1972) was in which parts of the disk are violently reset, while others confirmed using infrared spectroscopy. The characteriza- are inherited and preserved over the lifetime of the central tion of ices in the outer solar system continues to this day star, and where most regions show some evidence for both, (Mumma and Charnley 2011; Brown et al. 2012). Simi- due to a variety of mixing processes. In this chapter, we will larly, the presence of ices in the dense interstellar medium consider the solar nebula as a protoplanetary disk among – the material out of which all planetary systems form – many others – and indeed call it the solar protoplanetary has been recognized since the 1970s and conjectured even disk. We will discuss the issue of inheritance versus reset, earlier. With the advent of powerful infrared satellite ob- and suggest a division of disks into regions characterized by servatories, such as the Infrared Space Observatorory (ISO) these very different chemical circumstances. and the Spitzer Space Telescope, we know that interstellar ices are commonplace and carry a large fraction of the solid 1.2. Defining protoplanetary disks and volatiles mass in protostellar environments (Gibb et al. 2004; Oberg¨ The term protoplanetary disk generally refers to the ro- et al. 2011b). tationally supported, gas-rich accretion disk surrounding The next logical investigative steps include the tracking a young pre-main sequence star. The gas-rich disk per- of the volatiles as they take part in the formation and evolu- sists during planetesimal and giant planet formation, but tion of protoplanetary disks — the intermediate evolution- not necessarily during the final assembly of terrestrial plan- ary stage between the interstellar medium and evolved planets. During the lifetime of a protoplanetary disk, both solid- etary systems during which the planets actually form. This and gas-phase chemistry is active, shaping the initial com- subject, however, has seen little progress until recently. A position of planets, asteroids, comets and Kuiper belt ob- decade ago, volatiles in protoplanetary disks were essen- jects. Many of the chemical properties and architecture tially beyond our observational capabilities and the paths of the current day solar system were set during the nebu- the volatiles take from the initial conditions of a protoplan- lar/protoplanetary disk phase, and are indeed preserved to etary disk until they are incorporated into planets and plan- this day, although somewhat obscured by dynamical mix- etesimals were not well understood. What happened over ing processes during the later debris disk phase. the past decade, and in particular since the conclusion of Most of the mass in protoplanetary disks is in the form PPV, is that our observational knowledge of volatiles in pro- of molecular hydrogen and helium gas. Some of the solid toplanetary disks has been greatly expanded, opening up a mass is carried as dust grains, mostly composed of silicates new era of comparative cosmochemistry. That is, the so- and with some contribution of carbon-dominated material. lar nebula is no longer an isolated case study, but a data This material is generally refractory, that is, very high tem- point – albeit an important one – among hundreds. The peratures (& 1000 K) are needed to sublimate it. Only in emerging complementary study of volatiles in protoplane- a very limited region, in a limited period of time or under tary disks thus feeds on comparisons between the properties unusual circumstances are refractory grains returned to the of current-day solar system material and planet-forming gas gas phase. The opposite of refractory is volatile. In the con- and dust during the critical first few million years of the de- text of this chapter, disk volatiles are molecular or atomic velopment of exo-planetary systems. This is an area of in- species with relatively low sublimation temperatures (< a tense contemporary study, and is the subject of this chapter. few 100 K) that are found in the gas phase throughout a sigificant portion of a typical disk under typical, quiescent 1.1. Emerging questions disk conditions1. It is also possible to define a sub-class of Perhaps one of the most central questions in astronomy volatile material that includes all condensible species – that today is whether our solar system, or any of its charac- is, the class of volatiles that are found in both their solid and teristics, is common or an oddity? We already know that gaseous forms in significant parts of the disk, but which ex- most planetary systems have orbital architectures that do cludes species that never condense in bulk, such as H2. It not resemble the solar system, but is the chemistry of the is the ability of condensible volatiles to relatively easily un- solar system also uncommon? Another matter is the degree dergo dramatic phase changes that lead to them having a to which volatiles are inherited from the parent molecular special role in the evolution of protoplanetary disks and the cloud, or whether their chemistry is reset as part of typi- formation of planets. An example of a volatile that is not cal disk evolution. That is, can we recover evidence for considered condensible is the dominant mass compound, an interstellar origin in protoplanetary and planetary mate- H2, while one of the most important condensible species rial, or is that early history lost in the proverbial furnace of is water. planet formation? There is currently an apparent disconnect between the cosmochemical idea of an ideal condensation 1In cosmochemistry the use of the words refractory and volatile is some- sequence from a fully vaporized and hot early phase in the what different. While they are still related to condensation temperature, their use is usually reserved for atomic elements, rather than molecules, solar nebula, supported by a wealth of data from meteoritic for interpreting elemental abundances in meteorites. material, and the astrophysical idea of a relatively quiescent path of minimally processed material during the formation of protoplanetary disks. The answer is likely a compromise, 2 2. The solar nebula as a volatile-rich protoplanetary observable is illustrated in Figure 1, where the Earth’s rela- disk tive elemental abundances are compared to those of the Sun. While the refractory elements are essentially of solar abun- The solar system was formed from a protoplanetary disk dances, others are depleted by orders of magnitude, most during a time frame of 2-3 million years, spanning ages of prominently carbon and nitrogen, and to a lesser degree, at least 4567-4564 Myr – a number known to a high de- oxygen. Going beyond the Earth, C, N and O abundances gree of precision, thanks to accurate radiometric dating of in various solar system bodies are compared (Earth, mete- primitive meteoritic material formed by gas-condensation orites, comets, and the Sun) in Figure 2. processes (Scott 2007). The composition of solar system bodies, including primitive material in meteorites, comets, and planets provide a detailed window into their formation.

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